1. Field of the Invention
The present invention relates to a dielectric line for propagating an electromagnetic wave of a frequency within the range of 1 to 10 GHz, and an electronic component including the dielectric line.
2. Description of the Related Art
Nowadays, the microwave band, especially a frequency band of 1 to 10 GHz, is often used for near-field communications and mobile communications. It is strongly demanded that communication devices used for such communications be reduced in size and thickness, and accordingly, reductions in size and thickness are also strongly demanded of electronic components used for such communication devices.
Typically, transmission lines having a structure in which a conductor and a dielectric are combined, such as coaxial lines, strip lines, microstrip lines and coplanar lines, are used for transmission of high frequency signals in the 1- to 10-GHz frequency band.
Some electronic components, such as bandpass filters, used for communication devices include resonators. While some of the resonators use a distributed constant line and some others use an inductor and a capacitor, each of the resonators includes a transmission line. The resonators are required to have a high unloaded Q. The unloaded Q of a resonator can be increased by reducing losses in the resonator.
The losses in a transmission line include dielectric loss, conductor loss, and radiation loss. As the frequency of a signal increases, the skin effect becomes significant, and accordingly the conductor loss increases significantly. Most of the loss in a resonator is attributable to the conductor loss. Therefore, to increase the unloaded Q of the resonator, it is effective to reduce the conductor loss. Known techniques for reducing the conductor loss of a resonator to thereby increase its unloaded Q include those disclosed in JP H04-043703A and JP H10-013112A.
The technique disclosed in JP H04-043703A is as follows. In a symmetric strip line resonator, a plurality of strip conductors separated from each other by a dielectric are disposed between a pair of ground conductors so as to be parallel to the ground conductors. The conductor loss in the strip conductors is thereby reduced to increase the unloaded Q of the resonator.
The technique disclosed in JP H10-013112A is as follows. In a resonator having a strip line electrode, a multi-layer electrode including a conductor and a multi-layer section in which dielectric layers and conductor layers are stacked alternately is used as the strip line electrode. The multi-layer section is disposed such that surfaces of the layers constituting the multi-layer section are perpendicular to the plane of a ground electrode. The conductor loss in the strip line electrode is thereby reduced to increase the unloaded Q of the resonator.
On the other hand, a dielectric line is known as a transmission line for propagating electromagnetic waves in a millimeter wave band of about 50 GHz. For example, JP 2007-235630A discloses a transmission line having a configuration in which a high-dielectric constant tape is disposed between two parallel conductor plates arranged parallel to each other and a dielectric filler formed of a low-dielectric constant material is disposed between the high-dielectric constant tape and the two parallel conductor plates. In this transmission line, the electric field of the electromagnetic waves is distributed within the dielectric filler. JP 2007-235630A describes that an actually produced transmission line had low dispersion characteristics in a frequency band of 30 to 60 GHz.
JP 2013-045859A discloses a magnetic dielectric material that has good magnetic properties even in a GHz frequency band.
As described above, the conventional transmission lines for the 1- to 10-GHz frequency band each have a structure in which a conductor and a dielectric are combined. For such transmission lines, it is difficult to reduce conductor loss significantly even if measures are taken, such as increasing the surface area of the conductor as in the techniques disclosed in JP H04-043703A and JP H10-013112A. Accordingly, there is a limit in increasing the unloaded Q of resonators using such transmission lines.
On the other hand, although a dielectric line for propagating electromagnetic waves in a millimeter wave band of about 50 GHz is known as mentioned above, no dielectric line for propagating electromagnetic waves in the 1- to 10-GHz frequency band is known.
The wavelength of an electromagnetic wave is inversely proportional to its frequency. The wavelengths of electromagnetic waves in the 1- to 10-GHz frequency band are about 5 times to about 50 times the wavelengths of electromagnetic waves in a millimeter wave band of about 50 GHz. In general, the conventional dielectric line increases in size as the wavelength of an electromagnetic wave to be propagated therethrough increases. Accordingly, if the conventional dielectric line is used to form an electronic component such as a resonator for the 1- to 10-GHz frequency band, the resulting electronic component is large in size and therefore not practical.
Due to the wavelength-shortening effect of a dielectric, the wavelength of an electromagnetic wave propagating through a dielectric line is shorter than that of an electromagnetic wave propagating through a vacuum. However, the conventional dielectric line cannot provide a significantly high wavelength-shortening effect. By way of example, JP 2007-235630A describes that the dielectric filler has a relative permittivity of, e.g., 4 or less. Given that the relative permittivity is 4, the wavelength-shortening rate is 0.5. Consequently, the use of the conventional dielectric line could not significantly reduce the size of an electronic component by means of the wavelength-shortening effect of the dielectric.